The drive for alternative energy has increased development of photo-voltaic (“PV”) power plants that may be connected to the electrical grid. A PV power plant has an inverter which converts direct current voltage from a PV source, such as solar panels, to alternating current voltage suitable for connection to the electrical grid. As more and more large-scale PV power plants are put into operation due to the increasing green energy requirements, utilities are applying more and more regulations on PV power plants, since the quality and stability of a power system may be affected by the installation of PV power plants. Presently, some European countries such as France and Germany are requiring that PV power plants which are connected to a medium voltage grid are capable of performing a low voltage ride through (“LVRT”) operation. The LVRT operation means once the grid voltage dips 10% to 95% of its nominal value, any individual PV inverter in a PV plant can still output the same amount of current as before the grid voltage dip for a specified period of time.
Recently, as the capacity of single PV power plants has increased rapidly, the emphasis on high PV voltage installations is getting higher because such installations can lower the installation cost. An open circuit voltage of about 1000V is currently desired from many PV installers. Facing this challenge, PV inverter manufacturers have two options: use high-voltage switching devices such as 1700V insulated gate bipolar transistors (“IGBT”) to build the power conversion bridge for PV inverters; or use regular voltage switching devices such as 1200V IGBTs in order to acquire high power conversion efficiency in inverters. Use of high-voltage switching devices is relatively expensive because of the higher voltage requirements. However, there are several challenges in use of regular voltage switching devices such as 1200V IGBTs for 1000V PV systems, one of which is the implementation of LVRT in PV inverters which may expose such switching devices to higher voltages than their operational design.
In general, running 1200V IGBTs above 850V at full load is not recommended for the safety of the IGBTs and the inverter. However 1200V IGBTs may be theoretically used for a 1000V PV installation since the Maximum Power Point Tracking (“MPPT”) voltage of this kind of PV installation will be less than 850V and usually less than 700V. However, as the grid voltage dip occurs, the DC voltage of the PV inverter may jump higher than 850V due to the instant energy build-up at the DC side. The IGBTs in the power bridge may fail in this situation and therefore the PV plant cannot perform LVRT. Thus, it would be desirable to have an inverter with relatively lower voltage components that can perform LVRT.